Passive ventilation is frequently specified by a building designer. For spaces to be passively ventilated they must be connected to each other to allow passive (i.e. unforced) movement of air. Ultimately, the air pathways are connected to the outside to allow fresh air to enter the building and to allow circulated air to leave the building.
However, where free movement of air is permitted from one area to another, the air provides a mechanism for sound to be transmitted between the two areas. Allowing air to enter a building from the outside will allow sound from the outside to enter the building. Similarly, allowing air to pass from one internal space to another will allow sound to pass from one space to another. Clearly this is undesirable.
Typically, a passively ventilated building will work most effectively when air is able to move at or above a given rate between internal spaces and/or between outside and inside. In addition, the acoustic specification of a building will place limits on the acceptable transmission of sound between adjacent areas of the building. There is a conflict between the desire to allow free air movement and the desire to avoid ingress and egress of sound. The same issues can also arise in mechanical ventilation systems, for example when it is undesirable for the noise of a fan or other elements of an air conditioning system to be transmitted to a room without some attenuation.
Attempts have been made to address this issue in the past by providing sound attenuating air vents. Typically, such air vents will use an indirect air path lined with a sound absorbing material. The indirect air path is used to ensure that there is no direct path for sound and this has been considered essential in traditional sound attenuating air vent systems. “Periscope” type vents are known, and consist of a vent with a periscope shape, i.e. with two right angle bends, which is lined with a sound absorbent lining.